Patent GB-620488-A (GB)
Title: Improvements in or relating to radio direction indicating systems
IPC: G01S1/02
Filed: 1946-02-06
Abstract: 620,488. Radio navigation. O'BRIEN, W. J., and DECCA RECORD CO., Ltd. Feb. 6, 1946, No. 3724. [Class 40 (vii)] A radio directional indicating system of the kind in which two distinguishable fields are produced, one of which is rotating and the other provided for reference, and wherein the direction of the transmitters from the receiver is indicated by the phase difference between the signals received is arranged so that the rotating field rotates at a radio frequency and the reference field is either fixed or rotating in the same direction at a different frequency or in the opposite direction at any frequency. Further, the frequencies used are chosen so that the signals produced in the receiver by the two fields are each different multiples of a common frequency so that the phase comparison may be made by multiplying the frequencies of one or both the signals to bring them to a common comparison frequency. In an embodiment using a fixed reference field and in which the fields themselves are created at harmonically related frequencies (Fig. 1), a 60 kc. oscillator 1 feeds reference aerial A through amplifier 2 and also frequency conversion means 4 and 5 to provide a 90 kc. signal which is applied through two separate channels comprising phase controllers 6 and 7 and amplifiers 8 and 9 to two quadrantally spaced pairs of aerials B, C and D, E by means of which the rotating field is; produced. Pick-up loops 16 and 21 feed through multipliers to phase discriminators 18 and 23, also fed with signals from unit 4, which make phase comparisons at 180 kc. and provide control voltages fed to controllers 6 and 7 to maintain the required phase relation between the fields. The rotating field may be created by three or more aerials spaced in a circle and with a phase difference between the antenna feeds which is equal to the angle subtended at the centre of the circle by the are separating the antennae. At the receiver the 60 kc. reference signal and the 90 kc. signal are passed through separate channels and multipliers and their phase compared at 180 kc. by a phasemeter which indicates the bearing of the transmitters. As the frequency of the 90 kc. rotating field signal is multiplied by 2, the bearing indicated has a 180 degrees. ambiguity;- this might be eliminated by using frequencies having a two to one relationship, the rotating field being radiated at the higher frequency. In another embodiment, Fig. 6, the rotating field is obtained from a similar antenna system fed from 60 kc. oscillator 34 through quadrature network 37, 38, 39 and amplifiers 35 and 36. The field coils 40 and 41 of a goniometer are connected across the inputs to these amplifiers and the search coil 42 is rotated at a speed of 50 c.p.s. by motor 46 synchronized with 200 c.p.s. oscillator 45; the 60,050 cycle output from the search coil is amplified and modulated with the 200 o.p.s. signal before being radiated from antenna A to form the-reference field. At the receiver all the signals pass through a common channel to a square-law detector 50, the 50-cycle beat frequency between the rotating and reference field signals, which retains the rotating field phase information, is passed by filter 51, multiplied to 200 c.p.s. at 53 and applied to one input of phase discriminator 54, the other input of which is fed with the 200 c.p.s. reference frequency accepted by filter 52. A phase indication is given at 55 which is proportional to the bearing information required, but with quadrantal ambiguity. A bearing indicating system according to the invention may be duplicated to provide a fix for determining position or the system may be combined with the equi-phase contour system of Specification 620,479 in order to provide a less accurate, but less ambiguous system for "lane" identification. In this case a rotating field is intermittently created at one or more of the stations in place of the normal field system and the phase of the signal therefrom is compared with the phase of the master station, the phase relation between which and the signal previously radiated from the rotating field station has been previously determined by the system of the prior Specification. The master station A (Fig. 9) may radiate a 60 kc. signal generated by oscillator 56 and intermittently mbdulated with an audio frequency from Oscillator 59 fed over contacts 60 operated by a clockwork mechanism 62. At the spaced slave transmitter the signal is changed in frequency to 90 kc. and fed through switches 67, 68 to the transmitter associated with aerial B, a phasecontrolling path being provided; this produces a field as required by the system of Specification 620,479. When, however, the radiation from A is modulated, the modulation is detected by unit 77 and the signal produced operates relay 78 to change-over switches 67, 68, and so radiate a 90 kc. rotating field from antennµ B1, B2, B3 and B4 by means similar to those described with reference to Fig. 1. At the receiver the change in the phase difference indication provided by the navigation system receiver when the rotating field is being radiated provides the bearing information ; this may most easily be displayed by providing the phase-meter with a manually rotatable scale calibrated in azimuth. A bearing indication system using a rotating field and differently rotating reference field radiated from the same four aerials is described with reference to Fig. 10 (not shown). The rotating field-producing circuits are fed from a common source and each comprise frequency conversion means followed by phase splitting, amplifier and control circuits as shown in Fig. 1. The outputs of the two channels are connected in series to the feeds for each aerial pair. The direction in which the field rotates is dependent on the direction in which the phase advances round the aerial system and the rotating and reference fields may rotate in the same or opposite directions. At the receiver the phase comparison is made at the frequency which is the lowest common multiple of the frequencies used and the ambiguity in bearing is dependent on the sum of the multipliers used to bring the field frequencies to this common value. Specifications 620,480, 620,507, [both in Group XXXVI], arid 628,747 also are referred to.